1. Field of the Invention
The present invention relates to a multilayer filter including a plurality of resonators, each defined by a loop-shaped inductor and a capacitor electrode.
2. Description of the Related Art
Hitherto, a compact and inexpensive high-frequency band pass filter has been constructed by providing a plurality of LC resonators in a multilayer body, which is formed by stacking dielectric layers and electrode layers on top of one another.
Examples of such a multilayer band pass filter are disclosed in Japanese Unexamined Patent Application Publication No. 2006-067221 and International Publication No. WO 2007/119356.
Japanese Unexamined Patent Application Publication No. 2006-067221 discloses, as illustrated in FIG. 1 thereof, a three-stage multilayer filter including a jump-coupling capacitor C3 for achieving capacitive coupling between resonators in the first stage and third stage. Also, as illustrated in FIG. 3 of Japanese Unexamined Patent Application Publication No. 2006-067221, the jump-coupling capacitor C3 is defined by an electrode pattern 151 defining an inductor L1, an electrode pattern 153 defining an inductor L3, and an electrode pattern 161 which faces the electrode patterns 151 and 153.
However, in the configuration according to Japanese Unexamined Patent Application Publication No. 2006-067221, the electrode pattern 161 also faces an electrode pattern 152 defining an inductor L2, and thus an undesirable parasitic capacitance is generated between the electrode pattern 161 and the electrode pattern 152. This causes a problem of decreasing Q of the filter and degrading attenuation characteristics.
On the other hand, International Publication No. WO 2007/119356 discloses a configuration for decreasing a parasitic capacitance between an electrode pattern of a jump-coupling capacitor and a capacitor electrode pattern of an LC parallel resonator which is not coupled to the electrode pattern of the jump-coupling capacitor.
Here, the configuration of a multilayer band pass filter disclosed in International Publication No. WO 2007/119356 is illustrated in FIG. 1. The multilayer band pass filter illustrated in FIG. 1 includes a multilayer body which is defined by a ground electrode formation layer 701, a capacitor electrode formation layer 402, an input/output electrode formation layer 403, a strip electrode formation layer 404, and an external layer 405. The input/output electrode formation layer 403 is provided with an input electrode 721 and an output electrode 722, and an input-output capacitor electrode (an electrode pattern of a jump-coupling capacitor) 260. The input-output capacitor electrode 260 causes a capacitance to be generated between a capacitor electrode 411 connected to the input electrode 721 through via-electrode 441 and a capacitor electrode 414 connected to the output electrode 722 through via-electrode 442, and thereby causes the input electrode and the output electrode to be capacitively coupled to each other. The capacitor electrodes 411, 412, 413, and 414 of the capacitor electrode formation layer 402 face a ground electrode 409 which includes terminals 451 and 452.
To reduce the parasitic capacitance between the input-output capacitor electrode (an electrode pattern of a jump-coupling capacitor) 260 and the capacitor electrode 412 of a second-stage resonator, the capacitor electrodes of second-stage and third-stage resonators are displaced in the surface direction of the multilayer body with respect to the capacitor electrodes of first-stage and fourth-stage resonators.
The capacitor electrode 411, the ground electrode 409, via-electrodes 431 and 432, and a strip electrode 616 define a first-stage LC parallel resonator. The capacitor electrode 412, the ground electrode 409, via-electrodes 433 and 434, and a strip electrode 617 define a second-stage LC parallel resonator. The capacitor electrode 413, the ground electrode 409, via-electrodes 435 and 436, and a strip electrode 618 define a third-stage LC parallel resonator. Furthermore, the capacitor electrode 414, the ground electrode 409, via-electrodes 437 and 438, and a strip electrode 619 define a fourth-stage LC parallel resonator.
FIG. 2 is a schematic plan view illustrating the positional relationship of the four LC parallel resonators of the multilayer band pass filter illustrated in FIG. 1. The first-stage LC parallel resonator R1, the second-stage LC parallel resonator R2, the third-stage LC parallel resonator R3, and the fourth-stage LC parallel resonator R4 are disposed such that all the loop surfaces of the inductor electrodes thereof are parallel with one another.
According to the structure illustrated in FIG. 1, providing loop-shaped inductors produces an effect that the Q characteristics of the LC parallel resonators are improved and that the attenuation characteristics of the filter are improved.
Also, according to the structure illustrated in FIG. 1, the capacitor electrode of the second-stage resonator and the jump-coupling capacitor electrode do not overlap one another in perspective view in the stacking direction of the dielectric layers, and thus the parasitic capacitance therebetween can be decreased.
However, in the structures disclosed in Japanese Unexamined Patent Application Publication No. 2006-067221 and International Publication No. WO 2007/119356, in a case where three or more stages of LC parallel resonators are provided, the LC parallel resonators are disposed in a line such that the loop surfaces of all the LC parallel resonators are parallel with one another. Thus, there is a problem that, though electromagnetic coupling between the inductor electrode of each LC parallel resonator and the inductor electrode of the LC parallel resonator adjacent thereto can be adjusted, it is only possible to slightly adjust (set) electromagnetic coupling between the inductor electrode of the LC parallel resonator in the input stage and the inductor electrode of the LC parallel resonator in the output stage. This causes a problem that the degree of freedom of adjusting (setting) the attenuation characteristics of the filter (particularly, the position and band of an attenuation pole) is low.